1. Field of the Invention
This invention relates to a method of incorporating a four-wheel steering system in a vehicle, and a four-wheel steering system adapted to be incorporated in a vehicle by the method.
2. Description of the Prior Art
There has been proposed a four-wheel steering system in which both the front wheels and the rear wheels are turned in response to operation of the steering wheel. The four-wheel steering system is advantageous over the conventional two-wheel steering system in various aspects. For example, when the rear wheels are turned in the same direction as the front wheels (same phase) during high speed travel of the vehicle, the steering performance is improved to facilitate smooth lane changing since side force simultaneously acts on both the front and rear wheels. On the other hand, when the rear wheels are turned in the direction opposite to the turning direction of the front wheels (reverse phase) during low speed travel of the vehicle, the cornering performance is improved and the minimum turning radius of the vehicle is reduced.
The preferable rear wheel turning direction (the direction in which the rear wheels are turned for a given turning direction of the front wheels, that is, in the same direction, i.e., the same phase, or the reverse direction, i.e., the reverse phase) and the rear wheel turning angle ratio (the ratio of the turning angle of the rear wheels to the turning angle of the front wheels), vary depending on various factors such as the running conditions of the vehicle. (These characteristics are referred to as "the four-wheel steering characteristics", hereinbelow.) Accordingly, based on various concepts, there have been proposed various four-wheel steering systems in which the running condition of the vehicle is automatically detected and the four-wheel steering characteristics are automatically changed according to the running condition of the vehicle.
For example, it may be generally considered that the front wheels are not turned by a large angle when the vehicle runs at a high speed, and the time the front wheels are turned by a large angle is solely when the vehicle runs at a low speed. Accordingly, in one proposal, when the front wheel turning angle is relatively small, the rear wheels are turned in the same direction as the front wheels in order to, for instance, improve running stability, and when the front wheel turning angle is relatively large, the rear wheels are turned in the direction opposite to the front wheel turning direction in order to, for instance, improve cornering performance.
Further, the rear wheel turning mechanism may be mechanically connected to the front wheel turning mechanism so that the rear wheels are turned in response to operation of the steering wheel, or the rear wheel turning mechanism is electrically controlled so that the rear wheels are turned in response to operation of the steering wheel.
See, for example, U.S. Pat. Nos. 4,552,239, 4,601,357, 4,610,328 and 4,621,702, and Japanese Unexamined Patent Publication Nos. 61(1986)-108070, 61(1986)-8869, 62(1987)-8871, 62(1987)-8872, 62(1987)-12472, 62(1987)-18367, 62(1987)-12471 and 60(1985)-166561.
Since the rear wheels must be turned in response to turning of the front wheels according to predetermined fourwheel steering characteristics, the front wheel turning mechanism and the rear wheel turning mechanism must be adjusted with respect to each other when the four-wheel steering system is incorporated i a vehicle so that the rear wheels are actually turned according to the predetermined four-wheel steering characteristics, thereby making it difficult incorporation of the four-wheel steering system in the vehicle.
Further, since, in the four-wheel steering system, the front wheels and the rear wheels are operatively connected with each other, adjustment of toe-in of each of the front and rear wheels affect the toe of the other, thereby making it difficult incorporation of the four-wheel steering system in the vehicle.